What’s the days-to-physiological-maturity (DPM) rating for that new “Hybrid ABC” you’re planting this spring? Is it 100 days? Ninety-five days? Perhaps 104 days?

All three answers could be correct, depending upon where your farm is located. If planted in central South Dakota versus northern North Dakota, the same hybrid can differ by a week or more in terms of the number of days required for it to progress from emergence to physiological maturity. Why? Mainly on account of the variance in environmental conditions — particularly growing degree (heat) units — experienced in the respective locations over the course of the season. The same situation holds true for days-to-flower (DF) ratings.

Given this variability, farmers and crop insurers have, to a degree, been comparing “apples and oranges” when it comes to physiological maturity comparisons among sunflower hybrids. If one company bases its ratings upon tests conducted in South Dakota and another upon tests in North Dakota, the comparisons really are not accurate. The gap grows even wider if the High Plains is brought into the picture.

Discussions initiated two years ago by the National Sunflower Association Research Committee culminated in the establishment of a 1997 study to address this concern. The study’s three objectives were: (1) to identify a standard procedure for the classification of sunflower hybrids into maturity groups; (2) to identify criteria (e.g., a growth stage) which remained consistent across locations, planting dates and environmental conditions, thereby allowing hybrids to be consistently grouped according to their relative ranking; and (3) to identify certain commercial hybrids which could be used as standards to bracket the various maturity classes.

The study was coordinated by Dr. Arlo Thompson, crop manager-sunflower for Advanta, the parent company of Interstate Payco Seed Co. Participants included numerous commercial seed companies as well as USDA and university personnel in major sunflower-producing states.

Twelve commercial hybrids were chosen to represent the full range of sunflower maturities currently available in the North American market. These 12 hybrids were planted at 28 locations in seven states. Each cooperator was asked to collect data on (1) date of planting; (2) days from planting to emergence; (3) days from emergence to flowering;* and (4) days from emergence to physiological maturity.** Harvest moisture percentages were an optional measurement.

Complete sets of data were received from 15 of the 28 locations (seven in North Dakota, four in Nebraska, three in Minnesota and one in South Dakota). Those 15 locations thus comprised the basis for the study’s conclusions and groupings.

Thompson says averages from location to location differed considerably across the 12 hybrids in terms of both days to flower and days to physiological maturity. The most extreme variance was between Jamestown, N.D., and Glyndon, Minn., sites. While at a similar latitude, DF and DPM at these two locations differed by about three weeks due to a late planting date at Jamestown. “The significant differences among locations indicate that it is difficult to assign absolute values” for DF and DPM, Thompson states. “When we try to assign a number for ‘days to physiological maturity’ to a hybrid, that number can move 10 to 15 days, depending on where the hybrid is planted.”

The important point to remember, he continues, is that the rank order of a set of sunflower hybrids typically does not change — “whether they are planted in North Dakota or in Kansas. You just move the whole scale up or the whole scale down.” So if Hybrid ABC reaches physiological maturity four days earlier than Hybrid XYZ at a North Dakota site, it also can be expected to reach DPM four days sooner if the two are planted at the same site in Kansas. What will change is the actual number of days it takes for each hybrid to achieve physiological maturity at the respective sites.

Based on the 1997 study, the NSA Research Committee came up with a recommendation of four maturity groups incorporating four current hybrids as standards. This classification system — which has been endorsed by the NSA Board of Directors — is indicated in the box below. The system accommodates any current or future hybrids which may be earlier than the earliest hybrid in this study (Hysun 311).

“It’s not precise,” Thompson admits. “But it is precise enough to be able to say, ‘You have a Group 1, Group 2, Group 3 or Group 4 hybrid.’ It is based on the same format as the industry maturity grouping system used for many years in soybeans.” He reiterates that the relative rankings of hybrids should remain similar across location and environment, e.g., if a hybrid falls into Group 3 in Kansas testing, it likewise will be a Group 3 in North Dakota when grown next to the “check” hybrids.

The standardized grouping system is a voluntary effort by seed companies to provide more of an “apples to apples” method of comparison among hybrids. The system likely will be refined in the future (e.g., the addition of sub-groupings).

Thompson believes the new system should be a valuable tool for the crop insurance industry. Up until now, insurers have been utilizing company-supplied DPM data without knowing where (geographically) the data was generated. So a “95-day hybrid” rated in a southerly location could actually take 103 or 105 days to reach physiological maturity in a farmer’s field in northern North Dakota. The new grouping system will provide more accurate — and fair — determinations for crop insurance payments due to frost or other reasons.

Along with more-accurate crop insurance calculations, this system will give growers a way of measuring — across companies — how hybrids rank in terms of physiological maturity levels, he says.

Thompson knows producers also are very interested in at-harvest moisture percentage comparisons. That, however, is more difficult to classify. “Moisture of a particular hybrid may be influenced by a number of factors,” he points out, “including timing and extent of disease infection, plant standability, plant population and level of seedset in each plant.” Those factors can be extremely variable, thereby making at-harvest moisture comparisons (i.e., speed of drydown following achievement of physiological maturity) less accurate. — Don Lilleboe